1. Field of the Invention
This invention relates to a semiconductor light emitting device, and more particularly to improvement in a semiconductor laser which is suitable for use as a light source, for example, for optical communication.
2. Description of the Prior Art
FIG. 1 shows a known semiconductor laser device.
In FIG. 1, reference numeral 1 indicates an n type GaAs substrate; 2 designates an n type Ga.sub.0.7 Al.sub.0.3 As clad layer; 3 identifies a GaAs active layer; 4 denotes a p type Ga.sub.0.7 Al.sub.0.3 As clad layer; 5 represents a p type GaAs layer; 6 shows an electrode on the positive side; and 7 refers to an electrode on the negative side.
In a conventional semiconductor laser, as shown in FIG. 1, as the current increases the light emitting region spreads, resulting in a lateral oscillation mode becoming unstable. The reason for the instability is that there is no mechanism for stabilizing the lateral mode other than a difference in the gain of the current distribution.
In order to overcome this instability defect of the prior art, there has been proposed a semiconductor laser constructed as shown in FIG. 2, in which parts corresponding to those in FIG. 1 are identified by the same reference numerals.
The semiconductor laser of FIG. 2 differs from the semiconductor laser of FIG. 1 in that the clad layer 2 has a projecting portion 2G and in that the clad layer 2 outside of the projecting portion 2G is thinner than the corresponding layer of the FIG. 1 device. The cap layer 5 is composed of an n type GaAs portion 5' and a p type GaAs region 5" for defining therein a current path.
In the semiconductor laser of FIG. 2, light emitted from the active layer 3 travels out of the clad layer 2 outside of the projecting portion 2G and is absorbed and reflected by the n type GaAs substrate 1. That is, the effective refractive index in the portion outside of the stripe region, except the projecting portion 2G, is varied and the loss in that portion is increased. Therefore, the oscillation region is restricted to the portion corresponding to the projecting portion 2G which acts as an optical guide mechanism, and stabilizes the lateral oscillation mode.
However, the manufacture of this semiconductor laser poses some problems. The clad layer 2 is formed on the substrate 1 in which a recess is made prior to the formation of the clad layer 2. Since the clad layer 2 is very thin except for the projecting portion 2G, there is a possibility that the clad layer 2 sags in the area of the recess in the substrate 1, thus, causing the active layer 3 to curve. If the clad layer 2 is formed thick to avoid this problem, the light guide effect is lost. Another problem is the difficulty in obtaining the desired shape of the recess formed in the substrate 1. That is, in the case where a recess 1G is initially formed in the substrate 1 as shown in FIG. 6, and then the clad layer 2 is formed by liquid phase epitaxy on the substrate 1, as shown in FIG. 7, the edge of the recess 1G (indicated by the broken lines) is rounded into a gentle slope 1G'. The reason for this is as follows. During the formation of the liquid phase epitaxy layer, the layer flows over the edge of the recess 1G so, that when the growth solution makes contact with the substrate 1, the edge of the recess 1G is liable to be etched back into the solution. When the gentle slope 1G' is formed and the projecting portion 2G of the clad layer 2 also conforms to the recess 1G, the light emitting region becomes wider as current flows, thus making it impossible to control the region of oscillation. Still another problem is that it is very difficult to form the p type GaAs region 5" so that it is in alignment with projecting portion 2G. If they are not aligned, a current which does not contribute to increases in oscillation, to an increase in threshold current or the effective operating current, introduces non-uniformity in the light emission in the lateral direction and changes the light emitting region.
FIG. 3 illustrates another conventional semiconductor laser, constructed differently than the semiconductor laser of FIG. 2. The difference is that the n type GaAs substrate 1, of FIG. 3, is formed flat without a recess; a p type GaAs current preventing layer 8 is formed on the substrate 1; after a groove is formed in the layer 8, the clad layer 2 is grown on the layer 8 forming the projecting portion 2G in the groove.
This semiconductor laser has the same defects as those of the FIG. 2 device, except the current confinement problem. In addition, the p type GaAs current preventing layer 8, considered an advantage over the semiconductor laser of FIG. 2, is of no use in practice. In order for the laser device of FIG. 3 to serve as one having an optical guide mechanism, it is necessary that the p type GaAs current preventing layer 8 absorb light of the active layer 3 travelling out of the clad layer 2. Then, in the current preventing layer 8 electrons and holes are generated by the light absorption and only number of holes is gradually increased. This is equivalent to the application of forward bias voltage to the current preventing layer 8 with respect to the n type GaAs substrate 1 and the n type GaAlAs clad layer 2. When the number of holes has been increased, the current preventing layer 8 is biased to a potential substantially equal to a diffusion potential between the substrate 1 and the clad layer 2. When the current preventing layer 8, is not sufficiently thick as compared with the diffusion length of minority carries, electrons in the substrate 1 flow into the clad layer 2 through the current preventing layer 8, so that the current preventing layer 8 does not perform its function. The diffusion length of minority carriers varies with the carrier concentration in the range of 1 to 3 .mu.m for GaAs. To ensure that the current preventing layer performs its function, it must be between 5 to 10 times thicker than the diffusion length of minority carriers. It is difficult to achieve current preventing layer 8 thicknesses of up to 10 .mu.m and still form by means of etching a 6 .mu.m groove 2G, or to maintain the distance between the active layer 3 and the current preventing layer 8 in the range of up to 0.4 .mu.m, while keeping the active layer 3 flat. If the current preventing layer 8 and the active layer 3 are spaced a distance of 1 .mu.m or more so as to prevent the current preventing layer 8 from absorbing light, the layer 8 performs the current preventing function, but the optical guide function is lost.
A conventional semiconductor laser shown in FIG. 4 is also known in the art. In FIG. 4, parts corresponding to those in FIGS. 1, 2, and 3 are identified by the same reference numerals.
In FIG. 4, reference numeral 8 indicates a p type GaAlAs current preventing layer; and 9 designates a p or n type GaAs layer.
One of the defects of this semiconductor laser is an increase in the threshold current. That is, since a loss guide system in which light is absorbed by the GaAs layer 9 on the outside of the stripe region is used, light is guided only in the stripe, thus increasing threshold current. Another defect is that the active layer 3 becomes hollow and cannot be made flat, as shown in FIG. 5. The reason is that since the value l shown in FIG. 4 must be selected to be, for example, 0.3 .mu.m or less for guiding light, the active layer 3 is exposed directly to the influence of the groove. Still another defect is that since the GaAs layer 9 is thick, the gentle slope 1G', discussed with reference to FIGS. 6 and 7, is produced as in the cases of the other conventional devices.
At present, many studies are being made so as to overcome the abovesaid defects of the prior art. For example, there has been proposed to form the clad layer 2 to a thickness of up to 0.3 .mu.m (except the projecting portion 2G) and define the supersaturation degree of the growth solution, the cooling rate, the time for growing the active layer 3 and so forth, in order to grow the layer 3 flat. However, the manufacture of the semiconductor laser under such restricted conditions involves control difficulties. For example, an increase in the supersaturation degree of the growth solution suppresses the etching-back of the edge of the recess during the formation of the clad layer, but causes an increase in the growth speed. Accordingly, the clad layer tends to be thick, thus increasing the distance between the active layer and the substrate and resulting in the loss of the optical guide function.